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Original source: Filella, M., May, P.M., 2019. The aqueous solution thermodynamics of tantalum under conditions of environmental and biological interest. Applied Geochemistry 109, 104402.

Best stability constant values for tantalum species, 25 °C and I 0 mol L-1:

Reaction log K
Ta(OH)5(s) = Ta(OH)50 –5.295
Ta2O5(s) + 5 H2O = 2 Ta(OH)50 –20.00
Ta(OH)5 + H+ = Ta(OH)4+ + H2O 0.7007
Ta6O198– + H+ = HTa6O197– 16.35
HTa6O197– + H+ = H2Ta6O196– 14.00
Ta2O5(s) + 6 F + 4 H+ + H2O = 2 TaF3(OH)3 8.886
Ta2O5(s) + 10 F + 10 H+ = 2 TaF50 + 5 H2O 31.35
TaF5 + F = TaF6 3.003
TaF6 + F = TaF72– 2.129
TaF72– + F = TaF83– 2.374
TaF83– + F = TaF94– 2.673


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